Angel hates spiders. He usually stomps on them when Fluttershy isn't there to stop him, but this time she saved the spider from his fast and furious paws. No matter what Angel says or does, Fluttershy is intent on protecting this spider from him, and he can't have that.
Somepony needs to remember her place in the household, and if Angel has to get his paws dirty to remind her, he will.